The basic process of flash evaporation is described in U.S. Pat. No. 4,954,371 herein incorporated by reference. This basic process may also be referred to as polymer multi-layer (PML) flash evaporation. Briefly, a radiation polymerizable and/or cross linkable material is supplied at a temperature below a decomposition temperature and polymerization temperature of the material. The material is atomized to droplets having a droplet size ranging from about 1 to about 50 microns. An ultrasonic atomizer is generally used. The droplets are then flash vaporized, under vacuum, by contact with a heated surface above the boiling point of the material, but below the temperature which would cause pyrolysis. The vapor is cryocondensed on a substrate then radiation polymerized or cross linked as a very thin polymer layer. The material may include a base monomer or mixture thereof, cross-linking agents and/or initiating agents. A disadvantage of the flash evaporation method with radiation cross linking is that it requires two sequential steps, cryocondensation followed by curing or cross linking, that are both spatially and temporally separate. A disadvantage of this radiation crosslinking method is the time between cryocondensation and curing permitting the cryocondensed monomer to flow or run, especially on fixtures having irregular non-flat geometry, leading to non-uniformity of coating (FIG. 1a) so that the coating surface 150 is geometrically different from the substrate surface 160. Reducing surface temperature can reduce the flow somewhat, but should the monomer freeze, then cross linking is adversely affected. Using higher viscosity monomers is unattractive because of the increased difficulty of degassing, stirring, and dispensing of the monomer
The basic process of plasma enhanced chemical vapor deposition (PECVD) is described in THIN FILM PROCESSES, J. L. Vossen, W. Kern, editors, Academic Press, 1978, Part IV, Chapter IV-1 Plasma Deposition of Inorganic Compounds, Chapter IV-2 Glow Discharge Polymerization, herein incorporated by reference. Briefly, a glow discharge plasma is generated on an electrode that may be smooth or have pointed projections. Traditionally, a gas inlet introduces high vapor pressure monomeric gases into the plasma region wherein radicals are formed so that upon subsequent collisions with the substrate, some of the radicals in the monomers chemically bond or cross link (cure) on the substrate. The high vapor pressure monomeric gases include gases of CH.sub.4, SiH.sub.4, C.sub.2 H.sub.2, C.sub.2 H.sub.2, or gases generated from high vapor pressure liquid, for example styrene (10 torr at 87.4.degree. F. (30.8.degree. C.)), hexane (100 torr at 60.4.degree. F. (15.8.degree. C.)), tetramethyldisiloxane (10 torr at 82.9.degree. F. (28.3.degree. C.) 1,3,-dichlorotetra-methyldisiloxane) and combinations thereof that may be evaporated with mild controlled heating. Because these high vapor pressure monomeric gases do not readily cryocondense at ambient or elevated temperatures, deposition rates are low (a few tenths of micrometer/min maximum) relying on radicals chemically bonding to the surface of interest instead of cryocondensation. The low deposition rate is not useable in a high rate industrial application. Remission due to etching of the surface of interest by the plasma competes with deposition of the radicals. Lower vapor pressure species have not been used in PECVD because heating the higher molecular weight monomers to a temperature sufficient to vaporize them generally causes a reaction prior to vaporization, or metering of the gas becomes difficult to control, either of which is inoperative.
According to the state of the art of making plasma polymerized films, PECVD and flash evaporation or glow discharge plasma deposition and flash evaporation have not been used in combination. However, plasma treatment of a substrate using glow discharge plasma generator with inorganic compounds has been used in combination with flash evaporation under a low pressure (vacuum) atmosphere as reported in J. D. Affinito, M. E. Gross, C. A. Coronado, and P. M. Martin, A Vacuum Deposition Of Polymer Electrolytes On Flexible Substrates. "Paper for Plenary talk in A Proceedings of the Ninth International Conference on Vacuum Web Coating", November 1995 ed R. Bakish, Bakish Press 1995, pg 20-36., and as shown in FIG. 1b. In that system, the plasma generator 100 is used to etch the surface 102 of a moving substrate 104 in preparation to receive the monomeric gaseous output from the flash evaporation 106 that cryocondenses on the etched surface 102 and is then passed by a first curing station (not shown), for example electron beam or ultra-violet radiation, to initiate cross linking and curing. The plasma generator 100 has a housing 108 with a gas inlet 110. The gas may be oxygen, nitrogen, water or an inert gas, for example argon, or combinations thereof. Internally, an electrode 112 that is smooth or having one or more pointed projections 114 produces a glow discharge and makes a plasma with the gas which etches the surface 102. The flash evaporator 106 has a housing 116, with a monomer inlet 118 and an atomizing nozzle 120, for example an ultrasonic atomizer. Flow through the nozzle 120 is atomized into particles or droplets 122 which strike the heated surface 124 whereupon the particles or droplets 122 are flash evaporated into a gas that flows past a series of baffles 126 (optional) to an outlet 128 and cryocondenses on the surface 102. Although other gas flow distribution arrangements have been used, it has been found that the baffles 126 provide adequate gas flow distribution or uniformity while permitting ease of scaling up to large surfaces 102. In the method of radiation curing, a curing station (not shown) is located downstream of the flash evaporator 106. The monomer may be an acrylate (FIG. 1b). This system was for planar layer coatings. With radiation curing, the time between deposition and curing permits flow of thicker coating layers leading to non-uniformity of coating on non-uniform surfaces or tilted planar surfaces.
Therefore, there is a need for an apparatus and method for coating fixtures with polymerized layers at a fast rate while avoiding flow of the coating.